Fluorescent lighting systems of various types are commonly employed in a myriad of lighting applications. All fluorescent lighting systems are gas-discharge lamp systems and generate light by sending an electrical discharge through an ionized gas. The gas in any such system must be isolated from the surrounding atmosphere and electrically driven to a sufficient and sustained state of excitement to generate steady light. Therefore, all fluorescent lighting systems have at their heart a discharge envelope to isolate the gas mixture and a ballast circuit to excite the gas.
Discharge envelopes in the most common fluorescent lighting systems consist of a glass tube coated on the inside with an electroluminescent compound. The tube may be formed into a variety of shapes, such as a simple linear tube, a circular tube, and a wide variety of other formed tube shapes, including the increasingly familiar spiral form compact fluorescent lamp. The two ends of the tube are sealed to enclose a gas mixture commonly including a noble gas and mercury vapor. Electrodes are located at the ends in order to supply electric current to the gas mixture. While a variety of electrode forms are viable, electrodes in the ends of fluorescent discharge tubes commonly cathodes which take the form of a filament between two conductive pins which extend through the tube ends.
Ballast circuitry is necessary to provide both the initial ionization voltage to allow the arc between the electrodes in the discharge tube to strike, to continue to drive the gas mixture in the discharge tube to the correct sustained state of excitement, and to limit the current through the tube to a proper value for the tube. Depending upon the desired application, ballast circuitry may be integrated with the lamp discharge tube, or housed separately from a separable lamp.
Ballast circuitry for discharge lamps has in the past two decades undergone rapid advancement, isolating certain basic design concepts common to the majority of ballast circuits currently in production. The ballast circuitry generally incorporates an AC to DC rectification circuit, an ignition circuit, and bipolar transistors arranged in a self oscillating circuit. The rectification and ignition circuits vary little from design to design in the industry, and generally perform acceptably. However, the self oscillating circuits commonly used in electronic ballasts often perform undesirably as the lamp approaches the end of lamp life.
A basic self-oscillating ballast circuit passes all voltage in parallel through the discharge tube and a resonant capacitor. Where the first and second electrodes of the discharge tube take the form of a filament between two conductive pins, one or more electrode filament may fail and be open as the lamp approaches end of lamp life. The discharge between the electrodes no longer moves from filament to filament, but migrates instead to the pins. This situation significantly reduces the efficiency of the discharge and lamp and causes the electrodes to overheat.
In many discharge lamp applications, such as in compact fluorescent lamps or CFLs, the electrode ends of the discharge tube are in contact with a plastic housing. Though the overheated electrodes remain sealed within the discharge tube, they may reach temperatures in excess of 800 degrees Fahrenheit. This heat is conducted through to the exterior of the tube, causing the plastic to discolor, melt, and even burn, which is undesirable. Even when the electrode ends of the discharge tube are not in contact with a plastic housing, it is nevertheless undesirable to have superheated lamp tube surfaces.
An early solution to this problem was to connect the resonant capacitor through the filaments of the electrodes in series. When either filament opens, the resonant capacitor is disconnected, and circuit oscillation ceases. This operation prevents undesirable extreme overheating of the electrode due to filaments failing open near end of lamp life.
FIG. 2 shows a circuit diagram for an exemplary ignition and basic electronic ballast circuits portion for a discharge lamp incorporating this early solution. Voltage bus 201 and circuit common terminal 203 are respectively connected to or continued from positive bus wire 115 and negative bus wire 117 of FIG. 1. Voltage bus 201 is connected through charging resistor 205 to the anode of branch drawdown diode 209 and to first leads of diac 215, startup capacitor 211, and bleed resistor 213. Second leads of startup capacitor 211 and bleed resistor 213 are connected to circuit common 203. The cathode of drawdown diode 209 connects to half-bridge point 210. Pull-up supply resistor 207 bridges voltage bus 201 and half-bridge point 210. As described and shown, these components form ignition component 216 for an electronic ballast circuit.
A second lead of diac 215 connects to the base of first switching device or transistor 219. The collector of first switching device 219 is connected to half-bridge point 210. The emitter of first switching device 219 is connected to circuit common 203, and also through the series of first secondary winding 227 of base drive transformer 257 and first base current resistor 255 back to the base of first switching device 219.
Voltage bus 201 is connected to the collector of second switching device or transistor 221. Emitter of second switching device 221 is connected to half-bridge point 210. Half-bridge point 210 is also connected back to base of second switching device 221 through the series of second secondary winding 225 of base drive transformer 257 and second base current resistor 229. Snubber capacitor 217 bridges voltage bus 201 and half-bridge point 210 in parallel with collector and emitter of second switching device 221.
Voltage bus 201 is connected in series through DC blocking capacitor 231, to first pin 233 of discharge tube assembly 259. First pin 233 and second pin 235 are connected to the first and second leads respectively of first electrode filament 234, and support filament 234 at the correct location within the discharge tube. Second pin 235 is connected in series through resonant capacitor 251 to third pin 237 of discharge tube assembly 259. As with first electrode filament 234, third pin 237 and fourth pin 239 are connected to the first and second leads respectively of second electrode filament 238, providing similar support. Fourth pin 239 is connected to a first lead of resonant inductor 253. Second lead of resonant inductor 253 is connected in series through primary winding 223 of base drive transformer 257 back to half-bridge point 210.
In operation, two BJT transistors, 219 and 221, form a half-bridge inverter stage to output a square voltage wave. The square wave is snubbed by snubber capacitor 217 to reduce electromagnetic interference and transition stress on transistors 219 and 221, and other components. Resonant capacitor 251 and resonant inductor 253 form a resonant matching network to shape the snubbed output waveform, provide a proper starting voltage to strike an arc between cathodes 235 and 238 in discharge tube 259, and stabilize the lamp arc current during steady state operation. DC blocking capacitor 231 prevents DC voltage across resonant inductor 253, and additionally provides power to collector of transistor 221 during the oscillation cycle. The self-oscillating drive circuitry includes a base drive transformer 257 with one primary winding 223 and two secondary windings 225 and 227, and base current resistors 229 and 255. Base drive transformer 257 draws the resonant inductor current back through its primary winding 223 and is converted into a complementary voltage in the secondary windings 227, 255 to drive the bases of the respective transistors 221 and 219.
As lamp current and capacitor current are passed through the primary winding 223 of base drive transformer 257, the core of transformer 257 enters saturation and ceases to transfer power to the secondary windings. This saturation is desirable and assists the switching process, ending the base drive signal and rendering the transistors 219, 221 nonconductive.
An initial base pulse is provided to base of transistor 219 by ignition component 216. In operation, startup capacitor 211 is charged through charging resistor 205. Resistor 205 and bleed resistor 213 provide a divider to prevent overcharge of startup capacitor 211. Resistors 205 and 213 are typically of relatively high resistance values, with the resistance value of charging resistor 205 higher than that of bleed resistor 213. Once charge of capacitor 211 is sufficient to break down diac 215 to conductance, this first conductance provides the initial base pulse to transistor 219. Power to the collector of transistor 219 is provided during this initial cycle through initial supply resistor 207, which is typically of high resistive value. In subsequent cycles, power to base of transistor 219 is drawn from capacitor 211 through diode 209, with diode 209 operating to keep the charge on capacitor 211 below the breakdown value of diac 215 and preventing the re-firing of the initial base pulse.
The circuit described with reference to FIG. 2 is in common use in a variety of embodiments and variations, because it prevents oscillation of the circuit when a cathode filament 234, 238 fails open or when the lamp is removed from the ballast circuit. This arrangement, while improved, still exhibits undesirable characteristics because all current must flow through the cathode conduction loop. First, the circuit is inefficient. Directing all power through the electrodes causes significant power dissipation without producing light, and limiting the efficiency of the circuit's light output to around 85%. Additionally the increased thermal wear on the filament reduces lamp life.
Second, the end of lamp life protection on this circuit is limited. As a lamp 259 approaches end of lamp life, a failing filament 234, 238 may be of high resistive value while current may yet flow through it. Indeed, a filament 234, 238 may fail open and arc to complete the circuit. In these situations, power still flows through the cathode conductance loop and into the primary winding of base drive transformer, allowing oscillation to continue. This is undesirable as power is dissipated as heat at a rate much higher even than under normal operating conditions. The heat may reach potentially dangerous levels, and the efficiency of the lamp and ballast circuit is drastically reduced. Additionally, the ballast may cease to oscillate consistently, causing reduced light output or visually disturbing flickering and pulsing. These end of lamp life modes do not meet recognized end of lamp life protection standards.
A variety of end of lamp life protection solutions have been proposed. Few if any meet the trio of desirable characteristics—Efficiency, IEC qualification, and cost-effectiveness.
U.S. Pat. No. 5,019,751, granted May 28, 1991, to I. L. Flory, IV, et al., discloses a disabling circuit for deactivating a high pressure sodium lamp starting and operating circuit when the lamp exhibits end-of-life cycling includes a normally closed thermal switch connected to the starting and operating circuit is inoperative, the switch having contacts which open in response to an elevated temperature. A heating element is connected in parallel with the lamp so that the voltage across the lamp is applied to the heating element. The heating element is supported in a selected heat conducting relationship with the thermal switch so that a predetermined elevated temperature is reached and the contacts are opened only after the dissipation of an amount of energy resulting from repeatedly high lamp open-circuit voltage accompanying end-of-life cycling.
U.S. Pat. No. 5,475,284, granted Dec. 12, 1995, to J. N. Lester, et al., discloses a ballast that includes an inverter for providing an AC voltage to a discharge lamp. As the lamp approaches end-of-life, a DC voltage component develops across the lamp. The ballast includes circuitry for monitoring the condition of each of the cathodes by measuring this DC voltage component. After a predetermined increase in this DC voltage component, the inverter is disabled in order to prevent excessive heating of the cathodes.
U.S. Pat. No. 5,574,335, granted Nov. 12, 1996, to Y. Sun, discloses a ballast that includes an inverter for providing an AC voltage to a discharge lamp. As the lamp approaches end-of-life, a DC voltage component develops across the lamp. The ballast includes circuitry for monitoring the condition of each of the cathodes by measuring this DC voltage component. After a predetermined increase in this DC voltage component, the inverter is disabled in order to prevent excessive heating of the cathodes. The inverter is also disabled as a result of a resonant or near resonant mode condition of a tank circuit caused by an open circuit condition or a leaking lamp.
U.S. Pat. No. 5,925,986, granted Jul. 20, 1999, to M. Moisin, discloses an improved ballast circuit for controlling the power delivered to a fluorescent lamp. The present invention uses a complex resonating circuit to dynamically adjust the power being delivered to the load. The present invention also operates in burst mode allowing an increased voltage to be applied across the lamp load without overstressing the circuit. The increased voltage will light both lamps nearing the end-of-life and lamps in cold weather.
U.S. Pat. No. 6,366,032, granted Apr. 2, 2002, to J. M. Allison, et al., discloses a resonant fluorescent lamp ballast that includes an IC driver for a half-bridge arrangement that supplies AC current to a lamp. The IC creates a frequency sweep from a pre-heat frequency, through a resonant frequency, to a still lower operating frequency. A pre-heat pin in the IC triggers a re-start or the frequency sweep when a first signal exceeds a first threshold level. A shut-down pin in the IC associated with an internal shut-down latch shuts down the driver when a second signal exceeds a second threshold level. Pre-heat trigger circuitry detects a current spike through half-bridge switches when the lamp has not yet started, supplying the pre-heat pin with a first signal exceeding the first threshold level. End-of-life circuitry provides to the shut-down pin a second signal exceeding the second threshold level if lamp current fails to reach a substantial portion of its normal level within a predetermined time. A DC current-supply path is provided from a DC current supply, through at least one filament of each lamp in the load circuit, to a power-supply pin for the IC. The end-of-life circuitry cooperates with the pre-heat trigger circuitry by limiting the number of the frequency sweeps to no more than occur during the predetermined time set by the end-of-life circuitry. The DC path cooperates with the end-of-life circuitry and the internal shut-down latch to reset the latch when the DC path is broken due to absence of the at least one filament in the path.
U.S. Pat. No. 6,828,732, granted Dec. 7, 2004, to K. J. Yang, et al., discloses a fluorescent lamp end-of-life protection circuit in an illuminating electric appliance, including a rectifying effect protection circuit for a lamp tube and an overvoltage protection circuit for the lamp tube, in which a circuitry of two series resistors and a capacitor in series is connected in parallel with the lamp tube. A cathode of a transient voltage suppresser is connected to one end of the lamp tube, its anode is connected to an anode of a diode, and a cathode of the diode is connected to a common node of two series resistors. One end of a trigger diode is connected to a common node of the capacitor and the resistor, and the other end thereof is connected to a gate terminal G of a triac, a first electrode and a second electrode of which are connected to two ends of the lamp tube respectively.
U.S. Pat. No. 6,906,465, granted Jun. 14, 2005, to A. Cavallaro, discloses a self-ballasted fluorescent lamp that comprises a housing formed to contain a ballast. The housing has a top surface with openings leading from the exterior of the housing to the interior. The openings can be in the form of raised bosses having upstanding walls. A restricted portion, which can take the form of a flange, is formed with the opening and a safe end-of-life enabling device is positioned in the opening and in contact with the restricted portion. The safe end-of-life device is tubular, has low thermal conductivity and in preferred forms of the invention is a glass such as quartz or a ceramic such as aluminum oxide. The ends of a fluorescent lamp penetrate the tubular end-of-life device and extend into the interior of the housing for suitable connection to an enclosed electronic ballast. A retainer is positioned opposite the restricted portion for maintaining the end-of-life device within the opening. The retainer is a typical compact fluorescent cement that is used to fix the lamp to the housing. Alternatively, the retainer can be an adhesive silicone or a threaded washer.
Hence, the prior art recited above does not teach of the novel advantages that are found in the present invention.
What is needed is an improved ballast circuit that is responsive to a fluorescent lamp that is nearing end of lamp life and operating outside of its normal, efficient, and safe operating performance level.
Accordingly, it is therefore an object of the present invention to provide for a novel electronic ballast circuit to operate a compact fluorescent lamp that will turn off the lamp and ballast in both high impedance and open cathode end of lamp life failure modes by stopping oscillation of the ballast circuit. It is an additional object of the present invention to ensure that this end of lamp life protection is delivered in the most energy efficient manner.